Structural and optical properties of diamond and nano-diamond films grown by microwave plasma chemical vapor deposition

摘要

We compare structural and optical properties of microcrystalline and nanocrystalline diamond (MCD and NCD, respectively) films grown on mirror polished Si(100) substrates by microwave plasma chemical vapor deposition. The films were characterized by SEM, Raman spectroscopy, XRD, and AFM. Optical properties were obtained from transmittance and reflectance measurements of the samples in the wavelength range of 200-2000 nm. Raman spectrum of the MCD film exhibits a strong and sharp peak near 1335 cm~(-1), an unambiguous signature of cubic crystalline diamond with weak non-diamond carbon bands. Along with broad non-diamond carbon bands, Raman spectra of NCD films show features near 1140 cm~(-1), the intensity of which is significantly higher in the film grown at 600 ℃ compared to the NCD film grown at higher temperature. The Raman feature near 1140 cm~(-1) is related to the calculated phonon density of states of diamond and has been assigned to nanocrystalline or amorphous phase of diamond. XRD patterns of the MCD film show sharp peaks and NCD films show broad features, corresponding to cubic diamond. The rms surface roughness of the films was observed to be approximately 60 nm for MCD film that reduced substantially to 17 and 34 nm in the NCD films grown at 600 and 700 ℃, respectively. Tauc's optical gap for the diamond film is found to be approximately 5.5 eV. NCD grown at 700 ℃ has a high optical absorption coefficient in the whole spectral region and the NCD film grown at 600 ℃ shows very high transmittance (～78%) in the near IR region, which is close to that of diamond. This indicates that the NCD films grown at 600 ℃ has the potential for applications as optical windows since its surface roughness is significantly low as compared to the MCD film.